To obtain an inner focus type middle telephotographic lens where aberration is completely compensated and which is constituted of small number of lenses by constituting the lens of a 1st positive lens group, a 2nd negative lens group and a 3rd positive lens group in order from an object side and making the lens satisfy specified conditional expressions.

This lens is constituted of the 1st positive lens group 10, the 2nd negative lens group 20 and the 3rd positive lens group 30 in order from the object side. A diaphragm S is fixedly positioned between the 1st and the 2nd lens groups 10 and 20. Focusing is performed by moving the 2nd lens group 20. The lens satisfies a condition I: 1.0<f/f1<1.4 and a condition II: 1.0<f/f2<1.6. Provided that (f) is the focal distance of an entire system, f1 is the focal distance of the 1st lens group 10 and the f2 is the focal distance of the 2nd lens group 20. In order to restrain the occurrence of aberration, the power of the 1st lens group 10 is lowered first and then the power of the 2nd lens group 20 is lowered in accordance with the lowering of the power of the 1st lens group 10.